The present invention relates to a fuel cell system and a shutoff method for the system.
Development of fuel cell automobiles has prevailed in recent years, on which fuel cells such as Polymer Electrolyte Fuel Cells (PEFC) are mounted. A fuel cell automobile of this type is driven by a motor, to which electricity generated by a fuel cell is supplied.
Generally speaking, a fuel cell has a stack structure, which is made of a plurality of stacked unit cells. A unit cell has a membrane electrode assembly (MEA). When hydrogen and air containing oxygen are supplied to an anode and a cathode in the MEA, respectively, electric potential difference occurs in the unit cell. The fuel cell, which is electrically connected to an external load such as a motor, generates electricity.
The fuel cell described above produces water at the cathode while it is generating electricity. In order to maintain a moist condition for an electrolytic membrane of the MEA such that proton transportation is enhanced, it is often practiced that hydrogen and oxygen supplied during generation of electricity are humidified according to necessities.
It is feared that a fuel cell, which is possibly used in cold-temperature regions, encounters freezing if there remains water in the fuel cell. This freezing leads to blockades for gas passages in the fuel cell, which possibly cause a decrease in output electricity of the fuel cell during a subsequent startup.
A technique has been proposed that when generation of electricity by a fuel cell is shut off, water remaining in gas passages and on surfaces of MEA is forced to discharge into outside of the fuel cell by supplying a scavenging gas to the fuel cell (see patent document No. 1). In this connection, the scavenging gas represents a gas to be supplied to the fuel cell so as to forcefully discharge the water remaining in it. For example, air and nitrogen are used as scavenging gases. An operation for supplying a scavenging gas into a fuel cell so as to forcefully discharge water in the fuel cell is referred to as scavenging in this specification.    Patent document No. 1: Japanese Published Patent Application 2004-111196 (paragraphs 0019-0040 and FIG. 1).
However, although scavenging is conducted during a shutoff of the fuel cell, it sometimes occurs that water remaining on walls of the gas passages and in electrolytic membranes of the fuel cell happens to freeze eventually.
The applicants have discovered that there is a way to smoothly start up a fuel cell, which has experienced a temperature below the freezing point during its shutoff. For example, when it is feared that the fuel cell has frozen, hydrogen and air are supplied in high flow rate at a subsequent startup so that the fuel cell generates higher electricity. In this way, it is possible to rapidly warm up the fuel cell by its raised heat dissipation so as to increase its output electricity. In this specification, an operation is referred to as low-temperature startup, in which the fuel cell undergoes generation of electricity in a higher rate.
However, there has been a possible case where before it has elapsed sufficient time after the fuel cell is started in a low-temperature startup, an ignition (IG) is turned off and scavenging is conducted as usual at shutoff while the fuel cell keeps a high temperature. In this case, electrolytic membranes and the like are possibly dehydrated too much, which results in a possible decrease in output electricity of the fuel cell during its subsequent startup. There has also been a case where before it has elapsed sufficient time after the fuel cell is started in a low-temperature startup, IG is turned off while the fuel cell keeps a low temperature. Because water vapor (water), which is produced before turning off IG as a by-product in parallel with electricity generation, happens to condense in the fuel cell, the resulting condensed water possibly causes a decrease in output electricity of the fuel cell during its subsequent startup.